This invention relates to products which increase the density of wellbore fluids used during the construction or repair of oil, gas, injection, water or geothermal wells. The products of this invention may be used in any wellbore fluid such as drilling, cementing, completion, packing, work-over (repairing), stimulation, well killing, and spacer fluids.
One of the most important functions of a wellbore fluid is to contribute to the stability of the well bore, and control the flow of gas, oil or water from the pores of the formation in order to prevent, for example, the flow or blow out of formation fluids or the collapse of pressured earth formations. The column of fluid in the hole exerts a hydrostatic pressure proportional to the depth of the hole and the density of the fluid. High pressure formations may require a fluid with a specific gravity of up to 3.0.
A variety of materials are presently used to increase the density of wellbore fluids. These include dissolved salts such as sodium chloride, calcium chloride and calcium bromide. Alternatively powdered minerals such as barytes, calcite and hematite are added to a fluid to form a suspension of increased density. It is also known to utilise finely divided metal such as iron as a weight material. In this connection, PCT Patent Application WO85/05118 discloses a drilling fluid where the weight material includes iron/steel ball-shaped particles having a diameter less than 250 xcexcm and preferentially between 15 and 75 xcexcm. It has also been proposed to use calcium or iron carbonate (see for example U.S. Pat. No. 4,217,229).
It is a requirement of wellbore fluids that the particles form a stable suspension, and do not readily settle out. A second requirement is that the suspension should exhibit a low viscosity in order to facilitate pumping and to minimise the generation of high pressures. Another requirement is that the wellbore fluid slurry should exhibit low filtration rates (fluid loss).
Conventional weighting agents such as powdered barytes (xe2x80x9cbaritexe2x80x9d) exhibit an average particle diameter (d50) in the range of 10-30 xcexcm. To suspend these materials adequately requires the addition of a gellant such as bentonite for water based fluids, or organically modified bentonite for oil based fluids. A soluble polymer viscosifier such as xanthan gum may be also added to slow the rate of the sedimentation of the weighting agent. However, a penalty is paid in that as more gellant is added to increase the suspension stability, the fluid viscosity (plastic viscosity) increases undesirably resulting in reduced pumpability. This is obviously also the case if a viscosifier is used.
The sedimentation (or xe2x80x9csagxe2x80x9d) of particulate weighting agents becomes more critical in wellbores drilled at high angles from the vertical, in that sag of, for example, one inch (2.54 cm) can result in a continuous column of reduced density fluid along the upper portion of the wellbore wall. Such high angle wells are frequently drilled over large distances in order to access, for example, remote portions of an oil reservoir. In this case it becomes even more critical to minimise a drilling fluid""s plastic viscosity in order to reduce the pressure losses over the borehole length.
This is no less important in deep high pressure wells where high density wellbore fluids are required. High viscosities can result in an increase in pressure at the bottom of the hole under pumping conditions. This increase in xe2x80x9cEquivalent Circulating Densityxe2x80x9d can result in opening fractures in the formation, and serious losses of the wellbore fluid. Again, however, the stability of the suspension is important in order to maintain the hydrostatic head to avoid a blow out. The two objectives of low viscosity plus minimal sag of weighting material can be difficult to reconcile.
The need therefore exists for materials to increase fluid density which simultaneously provide improved suspension stability and less viscosity increase.
It is known that reduced particle sedimentation rates can be obtained by reducing the particle size used.
However, the conventional view in the drilling industry is that reducing the particle size causes an undesirable increase in viscosity. This is supposed to be caused by an increase in the surface area of the particles causing increased adsorption of water.
For example, xe2x80x9cDrilling and Drilling Fluidsxe2x80x9d Chilingarian G. V. and Vorabutor P. 1981, pages 441-444 states: xe2x80x9cThe difference in results (i.e. increase in plastic viscosity) when particle size is varied in a mud slurry is primarily due to magnitude of the surface area, which determines the degree of adsorption (tying up) of water. More water is adsorbed with increasing area.xe2x80x9d Further it is also stated that xe2x80x9cViscosity considerations often will not permit the addition of any more of the colloidal solids necessary to control filtration, unless the total solids surface area is first reduced by removing a portion of the existing claysxe2x80x9d. The main thrust of the argument is that colloidal fines due to their nature of having a high surface area to volume ratio will adsorb significantly more water and so decrease the fluidity of the mud. This is why they and others have recommended that it is necessary in weighted particulate muds to remove the fine solids to reduce viscosity. The same argument or concept is presented in xe2x80x9cDrilling Practices Manualxe2x80x9d edited by Moore pages 185-189 (1986). Also, the API specification for barite as a drilling fluid additive limits the % w/w below 6 xcexcm to 30% maximum in order to minimise viscosity increases.
It is therefore very surprising that the products of this invention, which comprise particles very finely ground to an average particle diameter (d50) of less than two xcexcmmicrons, provide wellbore fluids of reduced plastic viscosity whilst greatly reducing sedimentation or sag.
The additives of this invention comprise dispersed solid colloidal particles with a weight average particle diameter (d50) of less than 2 xcexcm and a defloculating agent or dispersant. The fine particle size will generate suspensions or slurries that will show a reduced tendency to sediment or sag, whilst the dispersant controlling the inter-particle interactions will produce lower rheological profiles. It is the combination of fine particle size and control of colloidal interactions that reconciles the two objectives of lower viscosity and minimal sag.
It is worth noting that small particles have already been used in drilling fluids but for a totally different purpose. Thus, EP-A-119 745 describes an ultra high density fluid for blow-out prevention comprised of water, a first and possibly second weighting agent and a gellant made of fine particles (average diameter from 0.5 to 10 xcexcm). The gelling agent particles are small enough to impart a good static gel strength to the fluid by virtue of the interparticle attractive forces. On the contrary, the present invention makes use of well dispersed particles: the interparticle forces tend to push away the other particles. If the concentration of small dispersed particles is sufficient, no gelling agent is needed.
According to the invention, a dispersant is added to the particulate weighting additive to allow it to find an acceptable conformation on the particle surface. This provides via a manipulation of the colloidal interactions rheological control, tolerance to contaminants and manipulation of the colloidal interactions rheological control, tolerance to contaminants and enhanced fluid loss (filtration) properties. In the absence of the dispersant a concentrated slurry of these small particles, would be an unpumpable paste or gel. According to a preferred embodiment of the present invention, the dispersant is added during the grinding or comminution process. This provides an advantageous improvement in the state of dispersion of the particles compared to post addition of the dispersant to fine particles. The presence of the dispersant in the comminution process yields discrete particles which can form a more efficiently packed filter cake and so advantageously reduce filtration rates.
According to a preferred embodiment, the dispersant is chosen so as it provides the suitable colloidal inter-particle interaction mechanism to make it tolerant to a range of common wellbore contaminants, including salt saturated.
According to a preferred embodiment of the present invention, the weighting agent of the present invention is formed of particles that are composed of a material of specific gravity of at least 2.68. This allows wellbore fluids to be formulated to meet most density requirements yet have a particulate volume fraction low enough for the fluid to be pumpable.
A preferred embodiment of this invention is for the weight average particle diameter (d50) of the new weighting agent to be less than 1.5 micron. This well enhance the suspension""s characteristics in terms of sedimentation or sag stability without the viscosity of the fluid increasing so as to make it unpumpable.
A method of comminuting a solid material to obtain material containing at least 60% by weight of particles smaller than 2 xcexcm is known for example from British Patent Specification No 1,472,701 or No 1,599,632. The mineral in an aqueous suspension is mixed with a dispersing agent and then ground within an agitated fluidised bed of a particulate grinding medium for a time sufficient to provide the required particle size distribution. An important preferred embodiment aspect of the present invention is the presence of the dispersing agent in the step of xe2x80x9cwetxe2x80x9d grinding the mineral. This prevents new crystal surfaces formed during the comminution step from forming agglomerates which are not so readily broken down if they are subsequently treated with a dispersing agent.
The colloidal particles according the invention may be provided as a concentrated slurry either in an aqueous medium or an organic liquid. In the latter case, the organic liquid should have a kinematic viscosity of less than 10 centistokes at 40xc2x0 C. and, for safety reasons, a flash point of greater than 60xc2x0 C. Suitable organic liquids are for example diesel oil, mineral or white oils, n-alkanes or synthetic oils such as alpha-olefin oils, ester oils or poly(alpha-olefins).
Where the colloidal particles are provided in an aqueous medium, the dispersing agent may be, or example, a water soluble polymer of molecular weight of at least 2,000 Daltons. The polymer is a homopolymer or copolymer of any monomers selected from (but not limited to) the class comprising: acrylic acid, itaconic acid, maleic acid or anhydride, hydroxypropyl acrylate vinylsulphonic acid, acrylamido 2-propane sulphonic acid, acrylamide, styrene sulphonic acid, acrylic phosphate esters, methyl vinyl ether and vinyl acetate. The acid monomers may also be neutralised to a salt such as the sodium salt.
It is known that high molecular weight polymers act as flocculants by bridging between particles while low molecular weight polymers for instance (less than 10,000) act as deflocculants by creating overall negative charges.
It has been found that when the dispersing agent is added while grinding, intermediate molecular weight polymers (in the range 10,000 to 200,000 for example) may be used effectively. Intermediate molecular weights dispersing agents are advantageously less sensitive to contaminants such as salt and therefore are well adapted to wellbore fluids.
Where the colloidal particles are provided in an organic medium, the dispersing agent may be selected for example among carboxylic acids of molecular weight of at least 150 such as oleic acid and polybasic fatty acids, alkylbenzene sulphonic acids, alkane sulphonic acids, linear alpha-olefin sulphonic acid or the alkaline earth metal salts of any of the above acids, phospholipids such as lecithin, synthetic polymers such as Hypermer OM-1 (trademark of ICI).
The colloidal particles comprise one or more materials selected from but not limited to barium sulphate (barite), calcium carbonate, dolomite, ilmenite, hermatite or other iron ores, olivine, siderite, strontium sulphate. Normally the lowest wellbore fluid viscosity at any particular density is obtained by using the highest density colloidal particles. However other considerations may influence the choice of product such as cost, local availability and the power required for grinding.
Calcium carbonate and dolomite posses the advantage that residual solids or filter cake may be readily removed from a well by treatment with acids.
This invention has a surprising variety of applications in drilling fluids, cement, high density fluids and coiled tubing drilling fluids to highlight a few. The new particulate weighting agents have the ability to stablise the laminar flow regime, and delay the onset of turbulence. It is possible to formulate fluids for several applications including coiled tubing drilling fluids, that will be able to be pumped faster before turbulence is encountered, so giving essentially lower pressure drops at equivalent flow rates. This ability to stabilise the laminar flow regime although surprising, is adequately demonstrated in heavy density muds of 20 pounds per gallon (2.39 g/cm3) or higher. Such high density muds using conventional weighting agents with a weight average particle diameter of 10 to 30 xcexcm would exhibit dilatancy with the concomitant increase in the presence drops due to the turbulence generated. The ability of the new weighting agent to stabilise the flow regime even in the presence of a component of larger particles, means that high density fluids with acceptable rheology are feasible with lower pressure drops.
A further and unexpected application occurs in cement whereby the new weighting agent will generate slurries of a more controlled and lower rheology so allowing it to be pumped more freely into position. The reduced particle size will tend to have a less abrasive nature, whilst its suspension characteristics will reduce the free water and other suspension issues encountered when setting the cement. The high fraction of fines should also act as efficient fluid loss control agents, so preventing gas migration and producing stronger cements.
The fluids of the present invention may also be used in non-oilfield applications such as dense media separating fluid (to recover ore for example) or as a ship""s ballast fluid.
The following examples are to illustrate the properties and performance of the wellbore fluids of the present invention though the invention is not limited to the specific embodiments showing these examples. All testing was conducted as per API RP 13 B where applicable. Mixing was performed on Silverso L2R or Hamilton Beach Mixers. The viscosity at various shear rates (RPM""s) and other rheological properties were obtained using a Fann viscometer. Mud weight were checked using a standard mud scale or an analytical balance. Fluid loss was measured with a saturated API fluid loss cell.
In expressing a metric equivalent, the following U.S. to metric conversion factors are used: 1 gal=3.785 liters; 1 lb.=0.454 kg; 1 lb./gal (ppg)=0.1198 g/cm3; 1 bbl=42 gal; 1 lb./bbl (ppb)=2.835 kg/m3; 1 lb/100 ft2=0.4788 Pa.
These tests have been carried out with different grades of barite: a standard grade of API barite, having a weight average particle diameter (D50) of about 20 xcexcm; a commercial barite (M) made by milling/grinding barite whilst in the dry state, with an average size of 3xcexc-5xcexc and colloidal barite according the present invention (with a D50 from 0.5 xcexcm to 1.5 xcexcm), with a dipsersant included during the xe2x80x9cwetxe2x80x9d grinding process. The corresponding particle size distributions are shown FIG. 1. The dispersant is IDSPERSE(trademark) XT (Mark of Schlumberger), an anionic acrylic ter-polymer of molecular weight in the range 40,000-120,000 with carboxylate and other functional groups. This preferred polymer is advantageously stable at temperature up to 200xc2x0 C., tolerant to a broad range of contaminant, gives good filtration properties and do not readily desorb off the particle surface.